1. Field of the Invention
The present invention relates to an apparatus for further compressing encoded digital video data streams for transmission and storage, and more particularly to a recording apparatus that is capable of recording an encoded digital video data stream in either its original form or a more compact form.
2. Description of the Prior Art
Numerous video bit-rate reduction coding schemes are known for compressing digitized video signals for transmission and storage at a reduced bit rate. Activities for developing these techniques are widely under progress. International standards have been created, while some are still under development, by organizations such as the ITU and ISO.
The motion-compensated transform coding algorithm is currently the most widely used method and has been adopted as the key technique by major international standards such as H. 261, MPEG1 and MPEG2 for compressing digital video sequences. A wide variety of literature on this coding method is available, an example of which is published in "The MPEG Video Compression Algorithm", Signal Processing, Image Communication, Vol. 4 No. 2, April 1992, pp. 129-140. Briefly, the algorithm works in the following manner. A current picture is divided into adjacent blocks, each containing a matrix of pixel data. For each block, a motion vector is detected by comparing the current block to a reference picture to find the best-matched prediction among possible blocks in the reference picture. The difference between the predicted block and the current block, i.e., the prediction error, is further compressed using an orthogonal transformation to remove spatial correlation. The transformed coefficients are quantized by a quantization parameter to discard the less important information. Finally, the motion vectors and quantization parameters are combined with the quantized coefficients and other side information for transmission or storage using fixed or variable length codes.
Another popular method for image compression is the subband/wavelet coding scheme, where an image is divided into a plurality of bands by using bandpass filters and each band is quantized and coded with a coder specifically adapted to it. In a fashion similar to the transform coding scheme, temporal redundancy is reduced by motion compensation. Motion-compensated prediction subbands are subtracted from the subbands of the original picture. The subband prediction errors are quantized by a quantization parameter and coded, together with the quantization parameters, motion vectors and other side information, before transmission or storage. An example of the subband coding scheme could be found in "Subband based TV coding", Signal Processing, Image Communication, Vol. 5, Nos. 1-2, February, 1993, pp. 105-118.
In both the image coding schemes described above and in other schemes, the encoded digital video data streams are in binary ("0" or "1") form. The total amount of "0"s and "1"s in the data streams per second, i.e., the bit rate, determines the bandwidth of the transmission channel or the volume needed in the storage medium for recording the data streams. In other words, a digital video sequence has to be compressed to a bit rate that is suitable for transmission through a particular channel or storage in a particular storage medium. A typical 74-minute compact disk, for example, can only store a 74-minute program if the bit rate of the data stream (including audio and system information) is 1.4 Mbps. In the MPEG1 international standard, video sequences are compressed to 1.15 Mbps, which is 1/140 of the original.
On the other hand, there are demands for recording more programs in a storage medium than in the normal recording mode. Most of the conventional analog video cassette recorders (VCR) are capable of multispeed operation which allows two or three tape speeds for longer recording/playing time, while sacrificing some quality of the transmitted video signal. For example, VHS recorders usually have 2-, 4-, and 6-hour playing times for the standard length T120 tape cassette. The corresponding tape speeds are 33.35, 16.67 and 11.12 mm/s, respectively. The servo system has provisions for changing the speed in accordance with a front-panel selector switch for the record mode of operation. For digital data streams, however, a longer recording time cannot be achieved by lowering the speed of the storage medium since the transmission rate of the data stream is constant. Lowering the speed of a writable compact disk below the normal recording speed, for example, would lead to imperfect recording of all the "0"s and "1"s of the received data streams. For a longer recording time capability, the encoded data streams have to be further compressed to a lower bit rate prior to recording. Another demand arises when the encoded digital programs are to be transferred or distributed to channels having a narrower bandwidth. A data stream that is encoded to 9 Mbps, for example, has to be further compressed to enable transmission through a 4 Mbps transmission channel.
Further compression of an encoded data stream could be achieved by first decompressing and reconstructing the encoded data stream, followed by the compression of the reconstructed sequences to a desired bit rate. This requires the apparatus to be equipped with a full decoder and a full encoder. Besides, both the decompressing and compressing processes will incur a long delay before the received data stream could be recorded or distributed. One of the techniques for manipulating compressed images can be found in "Algorithms for Manipulating Compressed Images," IEEE Computer Graphics & Applications, September 1993, pp. 34-42, which claims that image manipulation operations are performed 50 to 100 times faster without decorepressing and subsequently compressing the encoded data stream.